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Oerlikon L70
The Oerlikon L70 is an autocannon chambered in 20×110mm RB, first produced during the Second World War and one of the most widely manufactured autocannons in history. It is directly based on the German SEMAG L, itself a derivative of the Becker Type M2, the first autocannon to use advanced primer ignition (API) blowback operation. History After the collapse of the SEMAG company in the interwar years, the Zürich-based Oerlikon gun company bought up all rights to its designs. In 1927 they produced their own variant of the SEMAG-L, the Oerlikon S, the first step towards the Oerlikon L70. In 1935 the US Navy purchased two of the current model of Oerlikon for trials: they were rejected for their low muzzle velocity and rate of fire, almost bankrupting the company. Oerlikon was saved by coming up with a series of aircraft cannon versions of the design chambered for shorter 20mm cartridges, the Oerlikon FF, FFL and FFS. The Japanese Type 99-1 cannon and German MG FF were both derived from the Oerlikon FF, while the Japanese Type 99-2 variant was derived from the FFL. In 1937 the British Admiralty rejected the same version of the Oerlikon rejected by the USN two years earlier, but in 1938 stated that they would be interested if the muzzle velocity were improved and it were demonstrated that the weapon could be maintained by non-specialist personnel. First prototypes of a redesigned Oerlikon were delivered for evaluation in 1939, with the British sufficiently impressed that a large order was placed for production and a production license purchased. However, only about 100 Oerlikon-made guns were delivered before the Nazi invasion of France cut off the supply route. Undeterred, the British set up a factory at Ruislip and began manufacturing a domestic derivative, the Oerlikon Mark II, in the fall of 1941. The first British ship built with the Mark II was the battleship HMS Duke of York, which was fitted with six in November 1941. The gun was adopted by the US Navy in 1941 with a domestic Mark 2 design based on the original Swiss Mark 1. These were quickly mass-produced to replace the obsolete water-cooled M2 Brownings and 1.1"/75 caliber "Chicago Piano" quadmounts on US Navy vessels, replacing these weapons on a one-to-one or better basis. They proved hugely effective, with 32% of all Japanese aircraft shot down by the US Navy in the Pacific between December 1941 and September 1944 being claimed by Oerlikon guns. In 1943 USN Oerlikons started to be equipped with the state-of-the-art Mk 14 Gyro Gunsight, an electromechanical reflex sight with automatic target lead calculation. The Mk 14 was also used in standalone pedestal mounts as the Gun Director, Mk 51: in this application it was used to direct shipboard power-assisted gun mounts, either 1.1"/75 cal guns or more normally 40mm Bofors guns. Up to four guns could be slaved to a single director. The postwar Mk 20 was an improved version which was lighter, simpler, and decreased the Mk 14's three-minute warm-up time to ten seconds. They only began to lose effectiveness with the widespread use of kamikaze attacks late in the war, which led to the 40mm Bofors gun being preferred over then in 1944-45. By VJ Day, the United States alone had built 124,735 Oerlikons, and produced over 1 billion rounds of ammunition for them. The Oerlikon L70 was considered obsolete as a shipboard AA gun in the postwar period, and had been removed from most US warships by the mid-1950s. Smaller ships continued to carry them as direct-fire weapons, however: the Royal Navy did not retire their last Oerlikons from auxiliary vessels until 2006. The longer-barreled Oerlikon L85, also known as the Oerlikon KAA, was developed in the 1950s, but was not commonly employed until the Falklands War renewed interest in light warship armament. Design Details The Oerlikon L70 is a top-feeding, doward ejecting, air-cooled API blowback operated autocannon. The name comes from the barrel length of 70 calibers (70×20mm=140cm): some sources claim the British versions were L65, but gun data sheets show this is not the case. The original Swiss design was select-fire with a semi-automatic mode, but wartime models deleted this function as unnecessary and were auto-only. As a true API blowback weapon, the Oerlikon featured an elongated chamber and used rebated-rim cartridges so that the bolt head could grip the base of the cartridge while both entered the extended chamber. The primer was struck before the operating system was fully in battery, requiring the recoil forces to arrest the forward motion of the bolt before gas pressure could begin to push it backwards. In this way the Oerlikon could use a powerful cartridge without the impractically heavy bolt that a simple blowback system of the same caliber would require. The recoil spring required the application of 396 pounds (180 kg) of force to initially cock the gun. To accomplish this, a cocking rope was tied to the weapon's charging handle and passed over a pulley mounted on the gun cradle. The barrel was then fully elevated and the rope tied off to the base of the pedestal mount: lifting the gun back to horizontal by the shoulder rests would then provide sufficient leverage to easily cock the gun. Other options for cocking were a block-and-tackle set with a pull-handle and a ratchet system with a lever and chain, either of which could be bolted to the gun cradle. The gun had a bolt hold-open, meaning it did not need to be re-cocked after a reload. The gun was repeatedly redesigned during WW2 to simplify production: US design work eventually cut the 428.4 man-hours per gun of the original Swiss Mark 1 down to 76.2 man-hours per gun on their Mark 4 model. For example, the Mark 1's production process had the barrel spring housing start out as a 56 pound solid steel forging which then required 50 pounds of metal to be machined out of it to produce the final part: the US process instead started with a hollow-forged 14 pound base with a length of steel tube welded to it. The gun compared very favorably to the Hispano cannon, with the high operating forces meaning that factors such as elevation, weather and insufficient lubrication were unlikely to cause stoppages. The Oerlikon also had a fairly rapid barrel change for a gun of its size: the barrel could be swapped out in 30 seconds by a trained crew, while on the Hispano a barrel change was a major operation that took over an hour. Barrel lifespan during normal use was around 9,000 shots, and a barrel change would be required after running around six magazines through the weapon during a single bout of firing to prevent damage from overheating the barrel. The typical manual-traverse mounting had either one or two guns. The mount was operated by a single gunner with the rest of the crew acting as loaders: it had two shoulder pads and a belt for the operator's waist, with elevation controlled either manually or by a handwheel. More usually the handwheel was for height adjustment of the mounting so that it could be comfortably operated by crewmen of shorter stature. Free-swinging manual mounts were usually fitted with a rectangular splinter shield to protect the gunner, and a catching bag so that expended cases did not present a tripping hazard. In general, free-swinging gun mounts had full 360 degree traverse with no stops to prevent them accidentally firing at the superstructure of the ship they were mounted on: instead, the gun pit was designed to make it awkward to do so. Late-war naval mounts were often power-assisted and controlled via a joystick. Triple and quad mounts were also trialled, though the USN triple mount was cancelled over problems loading the middle gun, and the quad mounts were rarely used. Ammunition The Oerlikon L70 fired the [[20×110mm RB|'20×110mm RB']] round, a rebated-rim cartridge. A variety of different ammunition for the gun was developed during WW2: most rounds were explosive, though the British developed a saboted armor piercing (SAP) round for the gun and the USN had an armor-piecing tracer (AP-T) round. Wartime variants almost universally used a 60-round snail drum magazine. This drum was driven by a manually tensioned clock spring which was wound by turning the main shaft on the front of the drum using a separate tool, after cartridges were inserted. This "clockwork" drum operated in a similar manner to the drum of the Striker shotgun. A tension indicator was located on the front of the magazine, displaying 0, 15, 30, 45 or 60 to show how much tension was in the spring: 60 was for a full magazine, with crews instructed to tension for 10 more rounds than were in the magazine for partial loads. The drum was only fully tensioned if it was to be used immediately: otherwise it was stored partially tensioned to avoid damaging the springs and ratchet assembly. Loading and tensioning were performed below deck on warships, using a loading frame the magazine was placed on with its rear side facing down to prevent movement, so the loading crewman had both hands free. The loading procedure began by lifting the coupling sleeve (the projection on the front face of the magazine) and turning it until it rested on the lugs of the main shaft of the magazine, rather than being engaged with the ratchet mechanism as in normal operation. This allowed the main shaft to rotate disconnected from the driving spring, meaning the loader would not have to force rounds into the magazine against spring tension. Once the magazine was loaded, the coupling sleeve was rotated back into alignment with the rachet and the driving spring tensioned. Loading started by rotating the coupling sleeve until the follower was right at the mouth of the magazine. The loader then pushed the follower up into the magazine to create enough room for one or at most two rounds. They then inserted the rounds one at a time, sliding them down into the feed lips from above. After one or two rounds were loaded, the procedure was repeated, pushing the cartridges up into the drum and loading more. Attempting to rush this procedure could result in rounds jumping off the spiral feed tracks: if this occurred, the magazine would have to be dismantled to remedy it. A hand-lever loading tool that clamped over the mouth of the magazine was often required to insert the last few rounds due to the weight of those already inside. The tensioning tool could be used to wind back the follower if loading was proving overly difficult, though this required care as if it was moved too far the rounds closest to it could tip over and fall off the feed tracks. The standard version had the body of the drum overhang the gun to the right. Drums weighed 31 pounds empty: to assist in loading the magazine into the gun, vertical metal handles were provided on the front and rear of the magazine, allowing the gun crew to grip the handles and brace the drum against their chest while locating the feed column in the magazine well. The magazine is located by pins at its front and then rocked back to engage it with the magazine catch: this catch is pushed towards the front of the gun with the palm to release the magazine. In naval use they were typically loaded with 58 high explosive rounds first and then two practice cartridges with inert solid projectiles: this was done to prevent a high-explosive round detonating in the barrel if the muzzle cover was not or could not be removed. US Navy manuals stated this was able to penetrate the muzzle cover and up to one inch of additional ice in the barrel. Two rounds were used to ensure all debris was cleared from the bore before the first HE shell was fired. Twin mounts used specially made left-handed drums for their left-hand gun. These were marked with a a thick white zig-zag line on the body and the lifting handles also painted white, to ensure there was no confusion during loading. The British tested a belt-fed variant able to fire continuously at 150 rpm, but this was abandoned in favor of twin 40mm Bofors mounts. Variants Mark 1/Mark I Original Swiss production version. Extremely labor-intensive with extensive machining and hand-fitting, featured a semi-auto mode not found on any other version. Mark II Royal Navy domestic version. Mark 2 USN first domestic version, rearranged buffer springs, two locking slots and a barrel with cooling ribs. Mark 3 Decreased number of cooling ribs and only one locking slot. Mark 4 Most common version, deleted the barrel cooling ribs and redesigned the buffer to only use one spring. Reworked for Imperial rather than metric production measurements. Mod 4 version added a fluted chamber. [[Polsten|'Polsten']] Polish derivative of the L70, lightened and highly simplified with the number of parts reduced from 250 to 119. Trivia * After the USN liquidated its enormous stockpiles of Mark 4 guns in the 1950s, a few ended up on the US civilian market. These are NFA "double stamp" guns as they are both a machine gun and a Destructive Device. Each round of high-explosive ammunition for the gun is also a Destructive Device and requires its own paperwork and tax stamp. References * Ordnance Pamphlet No. 911, "20mm A.A. Gun," Department of the Navy Bureau of Ordnance, March 1943 Category:Autocannons Category:Mounted guns Category:Anti-aircraft weapons